Myocardial ischemia or angina can often occur in patients with coronary artery disease not only with exercise but with rest or with mental stress. Using rubidium positron emission tomography, myocardial blood flow can be measured, and this technique was used to investigate changes in blood flow in normal and ischemic myocardial regions, at rest and with exercise. These findings were compared to the number and duration of episodes of myocardial ischemia developed during normal daily living. With exercise, blood flow to the ischemic zones decreased in some patients and increased (insufficiently) in others. Patients who had decrease in blood flow with exercise to ischemic regions, suggesting vasoconstriction of the coronary vasculature with exercise, had more frequent episodes of ST segment change during normal daily life which occurred at low heart rates. In contrast, patients in whom blood flow increased in the ischemic zones with exercise had fewer episodes which occurred at a higher heart rates. These findings suggest that coronary vasoconstriction plays an important role in ischemia in patients with coronary artery disease, that this phenomenon occurs even with exercise and is responsible for ischemic episodes occurring at low heart rates.